The present invention relates to an image forming apparatus in which curling of a transfer sheet, happened immediately after fixing, is reduced.
An image forming apparatus is composed of an image carrier for carrying a toner image, an image writing section for writing an electrostatic latent image onto the image carrier, a developing section for developing the electrostatic latent image, a transfer section for transferring the toner image onto a transfer sheet, and a fixing device for fixing the toner image transferred onto the transfer sheet.
The fixing device incorporates a heating roller including a heating means such as a halogen lamp, and a pressure roller for pressuring the transfer sheet to the heating roller. When the transfer sheet travels through a nipping point formed by a contact area between the heating roller and the pressure roller, the toner is melted so that the toner is fixed onto the transfer sheet. When the transfer sheet has passed the nipping point of the heating roller and the pressure roller, the transfer sheet is curled due to the temperature difference on the surface in contact with the heating roller and the other surface in contact with the pressure roller. The curling of the transfer sheet tends to result in sheet jamming sheet during conveyance, and irregular alignment of the transfer sheets, when ejected. Therefore, the curl requires straightening after fixing.
In order to straighten the curled sheets, a special correcting device is used. Concerning the curl correcting device, one example incorporates paired rollers, being a hard roller and a soft roller, and the curled sheet is conveyed through the nipped section of the paired rollers, and thereby the curled sheet is straightened by a mechanical method. Another example incorporates a pressure roller having a hard surface, and a correcting roller having a soft surface which includes a heating means, whereby the curl resulting from the fixing device is forced to be an opposite direction (see Patent Document 1).
Patent Document 1: Official Gazette of Japanese Tokkaihei 6-3900
However, regarding the curl correcting device using the rollers described above, the paired rollers and a driving motor for rotating the paired rollers are necessary. Therefore, the mechanical structure of the curl correcting device requires a complicated mechanism, resulting in a rise in the cost. Further, there are also problem that a space for installing the correcting device is rarely prepared in the layout of the device, and thus to include the correcting device, the overall image forming apparatus size is increased.